This invention relates to a process for the thermal cracking of carbamic acid esters (urethane cracking) in which inert thermally stable high-boiling solvents having a defined boiling point or a narrow boiling range are used as heat exchange media.
A distinction can be made between the cracking of carbamic acid esters to form isocyanates in the gaseous and liquid phases and cracking in a fluidized bed. Cracking in the gas phase is described, for example, in EP-A-28,724; EP-A 100,047; EP-A 126,299; EP-A 126,300; EP-A 143,120; EP-A 261,604; EP-A 449,110; U.S. Pat. No. 3,734,941; and U.S. Pat. No. 3,870,739.
Cracking in the gas phase is a high-temperature process and is generally conducted at temperatures &gt;300.degree. C. in a vacuum of &lt;25 mbar. The cost of the gas phase cracking process technology, the thermal loading of the starting materials and products, the requisite prior evaporation of the carbamic acid ester, and the catalytic effects of metal surfaces which are still not completely understood make gas phase cracking less advantageous than cracking in the liquid phase. In particular, there is a risk of blockage in the evaporator region due to the formation of deposits because the problem of transferring out higher molecular weight secondary products has not been solved.
Cracking in a fluidized bed is described in EP-A 78,005, for example. Processes such as these have high energy requirements and appear to be difficult to implement on an industrial scale. Use of such fluidized beds on an industrial scale cannot therefore be foreseen due to this interim state of development.
Compared to gas phase cracking, cracking in the liquid phase may be carried out at lower reaction temperatures (i.e., temperatures &lt;300.degree. C.). However, rapid separation of the reaction products is necessary to prevent the back-reaction of the isocyanate and the hydroxyl component to form carbamic acid esters and to reduce or prevent the formation of resin-like by-products which can form deposits in the apparatus used. The formation of higher molecular weight secondary products can be reduced by dilution with an inert solvent. The solvent also transfers these by-product components from the apparatus.
Many of the known processes can be distinguished by the type of reactor employed. A stirred reactor is used in the process disclosed in EP-A 355,443. A thin-film or tubular reactor is used in the processes described in EP-A-61,013, EP-A 92,738, and EP-A 396,977. A reactor with a fitted column is used in the processes taught in EP 323,514 and EP-A 524,554. A combined cracking and rectification column is used in the process disclosed in EP-A 568,782. Reaction columns are used in the process described in EP-A 542,106.
Another distinguishing feature of the known cracking processes is the presence or absence of a solvent during the cracking reaction.
Solvent-free cracking is described in EP-A 355,443, EP-A 568,782, EP-A 966,925 and EP-A 524,554. One disadvantage of such processes is that cracking proceeds in the column bottom, i.e. in the evaporator. In this heated region there is the risk of by-product formation due to the severe temperature gradients. In order to remove these by-products, high proportions (15 to 25% by weight) of the reactor charge have to be transferred out. Otherwise, caked deposits can occur which can result in a blockage of the reactor.
The problem of caked deposits is curbed by the addition of solvent (EP-A 61,103, EP-A 92,738, EP-A 323,514, and EP-A 542,106). The best yields are obtained when cracking is conducted in the stripping part of a combined cracking and rectification column. In this type of apparatus, the cracking reaction is prevented from proceeding in the evaporator region with the aid of a suitable high-boiling solvent. This solvent transfers the heat energy from the evaporator into the reaction zone by evaporation and condensation.
Cracking of a carbamic acid ester can be conducted in a cracking and rectification column so that no carbamic acid ester comes into contact with the heated surfaces of the evaporator. The apparatus can be operated in this manner for long periods. In contrast to the columns described in DE-A 4,231,417 and in EP-A 0,524,554, only a slight outward transfer of the column bottom content is necessary because no carbamic acid ester, cracking product or by-products can be detected analytically at the bottom of the column.
This process enables complete cracking (free from by-products) of the carbamic acid ester to be achieved in the distillation part of the column. Consequently, losses in yield are prevented and subsequent work-up is considerably simplified. Secondary reactions and caked deposits in the column bottom are prevented because reacting products do not reach the column bottom at all. Service lifetime of the apparatus is definitely prolonged.
The carbamic acid esters to be used in the process according to the invention are compounds corresponding to the general formula R.sup.1 (NHCOOR.sup.2).sub.n, in which
R.sup.1 is an aliphatic hydrocarbon radical containing a total of from about 4 to 12 carbon atoms and, optionally, bearing inert substituents; a cycloaliphatic hydrocarbon radical containing a total of from about 6 to 15 carbon atoms and, optionally, bearing inert substituents; an araliphatic hydrocarbon radical containing a total of from about 7 to 10 carbon atoms and, optionally, bearing inert substituents, or an aromatic hydrocarbon radical containing a total of from about 6 to 15 carbon atoms and, optionally, inert substituents; PA0 R.sup.2 is an aliphatic hydrocarbon radical containing from about 1 to 20 carbon atoms, a cycloaliphatic hydrocarbon radical containing from about 5 to 15 carbon atoms or an aromatic hydrocarbon radical containing from about 6 to 15 carbon atoms and PA0 n is an integer of from 2 to 5. PA0 R.sup.1 is an aliphatic hydrocarbon radical containing a total of from 4 to 12 and, more preferably, from 5 to 10 carbon atoms; a cycloaliphatic hydrocarbon radical containing from 6 to 15 carbon atoms; a xylylene radical or an aromatic hydrocarbon radical containing a total of from 6 to 15 carbon atoms and, optionally, bearing methyl substituents and/or methylene bridges; PA0 R.sup.2 is an aliphatic hydrocarbon radical containing from 1 to 6 and, more particularly, from 1 to 4 carbon atoms; a cyclohexyl radical; or a phenyl radical; and PA0 n is an integer of from 2 to 4. PA0 R.sup.1 is the hydrocarbon radical linking the isocyanate groups of 1,6-diisocyanatohexane, 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethyl cyclo-hexane, 2,4-diisocyanatotoluene, 2,6-diisocyanatotoluene, 2,2'-, 2,4'- or 4-4'-diisocyanatodiphenyl methane, 2,4'- or 4,4'-diisocyanatodicyclohexyl methane or 1,5-diisocyanatonaphthalene and PA0 R.sup.2 is a C.sub.1-4 alkyl radical. PA0 1-(butoxycarbonylamino)-3,3,5-trimethyl-5-(butoxycarbonylaminomethyl)-cyclo hexane, PA0 1-(methoxycarbonylamino)-3,3,5-trimethyl-5-(methoxycabonylaminomethyl)-cycl ohexane, PA0 1-methyl-2,4-bis-(methoxycarbonylamino)-benzene, PA0 1-methyl-2,6-bis-(methoxycarbonylamino)-benzene, PA0 1-methyl-2,4-bis-(butoxycarbonylamino)-benzene, PA0 1-methyl-2,6-bis-(butoxycarbonylamino)-benzene, PA0 1,10-bis-(methoxycarbonylamino)-decane, PA0 1,12-bis-(butoxycarbonylamino)-dodecane, PA0 1,12-bis-(methoxycarbonylamino)-dodecane, PA0 1,12-bis-(phenoxycarbonylamino)-dodecane, PA0 1,3-bis-(ethoxycarbonylaminomethyl)-benzene, PA0 1,3-bis-(methoxycarbonylamino)-benzene, PA0 1,3-bis-(methoxycarbonylamino)-methyl)!-benzene, PA0 1,3,6-tris-(methoxycarbonylamino)-hexane, PA0 1,3,6-tris-(phenoxycarbonylamino)-hexane, PA0 1,4-bis-(ethoxycarbonylamino)-butane, PA0 1,4-bis-(ethoxycarbonylamino)-cyclohexane, PA0 1,5-bis-(butoxycarbonylamino)-naphthalene, PA0 1,6-bis-(methoxycarbonylamino)-hexane, PA0 1,6-bis-(ethoxycarbonylamino)-hexane, PA0 1,6-bis-(butoxycarbonylamino)-hexane, PA0 1,5-bis-(methoxycarbonylamino)-pentane, PA0 1,6-bis-(methoxymethylcarbonylamino)-hexane, PA0 1,8-bis-(ethoxycarbonylamino)-octane, PA0 1,8-bis-(phenoxycarbonylamino)-4-(phenoxycarbonylaminomethyl)-octane, PA0 2,2'-bis-(4-propoxycarbonylaminophenyl)-propane, PA0 2,4'-bis-(ethoxycarbonylamino)-diphenyl methane, PA0 2,4-bis-(methoxycarbonylamino)-cyclohexane, PA0 4,4'-bis-(ethoxycarbonylamino)-dicyclohexane methane, PA0 2,4'-bis-(ethoxycarbonylamino)-diphenyl methane, PA0 4,4'-bis-(methoxycarbonylamino)-2,2'-dicyclohexyl propane, PA0 4,4'-bis-(methoxycarbonylamino)-biphenyl, PA0 4,4'-bis-(butoxycarbonylamino)-2,2'-dicyclohexyl propane, PA0 4,4'-bis-(phenoxycarbonylamino)-dicyclohexyl methane and PA0 4,4'-bis-(phenoxycarbonylamino)-diphenyl methane. PA0 a) under the conditions of cracking, they substantially dissolve both the carbamic acid esters used as starting materials and the secondary products of the isocyanates which are formed as by-products of the reaction; PA0 b) they are substantially thermally stable under the conditions of cracking; PA0 c) they are substantially chemically inert to the carbamic acid esters used and to the isocyanates formed; PA0 d) they are substantially distillable under the conditions of cracking; PA0 e) they can be substantially separated by distillation from the reaction by-products; and PA0 f) they can be recycled.
The carbamic acid esters preferably used in the process according to the invention are those corresponding to the above formula in which
Particularly preferred carbamic acid esters for the process according to the invention are those corresponding to the general formula EQU R.sup.1 (NHCOOR.sup.2).sub.2
in which
Examples of suitable carbamic acid esters are
The "butoxy groups" mentioned are iso- and n-butoxy groups.
Solvents which are suitable for conducting cracking in columns of this type may be liquid or solid. Examples of such solvents are given in EP-A 542,106. The boiling points of suitable solvents under the conditions of pressure in the bottom of the column are at least 10.degree. C., preferably at least 40.degree. C., above the boiling points of the isocyanates and alcohols which form the basis of the carbamic acid esters that are to be cracked. These solvents satisfy the following requirements:
U.S. Pat. No. 3,919,278; EP-A 323,514; EP-A 61,013; and EP 92,738 disclose specific examples of high-boiling substances which satisfy these requirements. These disclosed solvents can be used in the practice of the present invention after they have been purified. Examples of other suitable solvents include the various isomeric benzyl toluenes, terphenyls, phenoxybiphenyls, phthalic acid di(ar)alkyl esters and o-phosphoric acid tri(ar)alkyl esters with 1 to 10 carbon atoms in the (ar)alkyl esters in each case, and mixtures of compounds of this type.
Technical dibenzyl toluene, benzyl-n-butyl phthalate, technical terphenyl and partially hydrogenated terphenyls, phenoxybiphenyls and isomeric mixtures thereof are particularly suitable for use in cracking and rectification columns. However, commercially available high-boiling solvents or heat transfer media do not exhibit a defined boiling point but exhibit a boiling range. This results in the separation by distillation of the solvent mixture in the cracking column and in a broad temperature profile. In the extreme case, low-boiling constituents are distilled off with the isocyanate or alcohol cracking products and the higher-boiling products become concentrated in the bottom of the column.